The question of whether apes menstruate is a common point of curiosity. Reproductive cycles are a fundamental feature of mammalian life, yet their specific manifestation varies greatly across species. Understanding the reproductive patterns of apes and other primates requires a clear biological distinction between the two main types of uterine cycles: the menstrual cycle and the estrous cycle. This comparison highlights a unique evolutionary trajectory within a select group of mammals.
Defining Menstruation and Estrous Cycles
The fundamental difference between the estrous and menstrual cycles lies in the fate of the endometrium, the tissue lining the uterus. In the estrous cycle, common to most mammals, the uterine lining thickens in preparation for pregnancy. If fertilization does not occur, this tissue is reabsorbed by the body.
The menstrual cycle, by contrast, is defined by the programmed shedding of this uterine lining when pregnancy does not happen, resulting in the discharge of blood and tissue. The key biological mechanism driving menstruation is spontaneous decidualization, which is the preparation and differentiation of the uterine lining regardless of whether an embryo is present. This specialized preparation means that, in the absence of a conceptus, the lining is broken down and expelled.
The Primate Species That Menstruate
All species of Great Apes exhibit true menstruation, including humans, chimpanzees, gorillas, and orangutans. The menstrual cycle is a characteristic feature of higher primates, specifically the catarrhines, a group encompassing Old World monkeys and apes.
Several species of Old World Monkeys also menstruate, such as rhesus macaques, baboons, and mangabeys. While menstruation is common in these simians, it is absent in lower primates like strepsirrhines, which include lemurs. True menstruation is a rare phenomenon in the animal kingdom, occurring in only a few non-primate species such as certain bats, the elephant shrew, and the common spiny mouse.
Evolutionary Context of Menstruation in Primates
The evolution of menstruation is closely linked to the development of spontaneous decidualization in the primate lineage. This process allows the uterus to prepare for implantation without requiring a hormonal signal from an embryo, which is necessary in most other mammals. This spontaneous preparation evolved due to an adaptive need for increased selectivity over the implanted embryo, sometimes referred to as the “choosy uterus” theory.
In primates, the embryo is highly invasive, and the spontaneous differentiation of the uterine lining may function as a defense mechanism. By preparing the decidua in advance, the mother gains greater control over the implantation process, potentially rejecting defective or genetically impaired embryos. An alternative idea suggests that shedding the lining is simply a mechanistic consequence of this spontaneous preparation and is metabolically less costly than indefinitely maintaining a fully prepared uterine lining.
Observable Differences in Ape Cycles
While the underlying biological mechanism is shared across Great Apes, the cycles show observable differences in length and external manifestation. The average human menstrual cycle is approximately 28 days, but non-human ape cycles are often more regular and sometimes longer. Chimpanzees typically have a cycle of about 37 days, while gorillas average 30 to 32 days, and orangutans average 29 days.
Menstrual bleeding in most non-human primates is minimal compared to humans. Gorillas are noted to have bleeding that is more copious and easily visible than other great apes, with a period lasting two to three days. In contrast to some Old World Monkeys that display pronounced sexual swelling during estrus, menstrual bleeding in apes is often subtle and not externally advertised.